1. Field of the Invention
The invention relates to a short inking unit for a rotary printing machine, and to a method of improving the ink splitting in such a short inking unit.
2. Description of the Related Art
For the purpose of inking plate cylinders in web-fed or sheet-fed offset printing machines, use is made of inking units, generally ductor inking units in sheet-fed offset printing and ductor-less film inking units in web-fed offset printing. As is known, such inking and damping-roll arrangements comprise a large number of rolls for the supply and metering of the ink, rolls for the spreading/distribution of the ink and transport of the ink by means of ink splitting and ink application to the printing plate. These “long” inking units, as they are known, permit frequent splitting and distribution of the conveyed ink layer, so that after the ink applicator rolls, a smooth thin ink film is produced.
Critical parameters for the ink transfer by means of ink splitting are molecular, physical interface effects such as cohesion and adhesion forces, surface tension, wetting, which interact closely.
At each roll contact, the ink film thickness is divided. In simplified form, the complex theory of ink splitting states that: if two rolls roll on each other, only one of which is inked, then the printing ink is distributed in a specific ratio onto both roll surfaces. The aim of the ink transfer is a uniform distribution of the printing ink picked up by the ink ductor or the film roll and optimum inking of the printing plate.
As a result of the large number of rolls, the intention is therefore to ensure good distribution of the printing ink on the plate cylinder and to produce an ink layer of defined thickness on the plate cylinder, the ink layer having a high uniformity even in continuous printing; faults which are inherent in the system, such as ghosting behavior, are intended to be attenuated in this way. The number and circumference of the ink applicator rolls have an important influence on the uniform inking of the printing plate. It is simpler to apply a thin ink film to the printing plate successively from a plurality of ink applicator rolls and to smooth this film, than to apply the same quantity of ink with fewer rolls. However, as a result of the large number of rolls, the inking unit becomes more complicated.
A further problem is the filling characteristic of the inking unit; the more ink applicator and ink transfer rolls the inking unit has, the longer the time taken until the ink applicator rolls discharge the desired quantity of ink to the plate cylinder, so that a large amount of waste is produced until the inking unit has reached its steady state.
Short inking units are also used in web-fed offset printing machines for newspaper printing. Short inking units have already been known for a long time, for example see U.S. 2002/0014171. These inking units have a small number of rolls. Because of the small number of splitting points for the ink splitting between the rolls, the resultant disadvantage is that the printing ink can only poorly be emulsified with the damping solution. Furthermore, the consistency of the ink is destroyed by excessive splitting back of ink into the ink fountain. This often leads to a cloudy image and a disrupted ink and water equilibrium. Current short inking units also have very few possibilities in layer thickness variation.
On the other hand, in the case of digital printing machines in particular, it is desirable to carry out actions to optimize the printing quality, primarily in the digital data set, without influencing the mechanical components within the printing machine, such as is done for example by setting screws or by means of run-up curves. In a printing machine of this type, oriented towards the image data, the actions are therefore carried out in the data at the prepress stage or in the raster image processor (RIP). An inking unit to be developed for such a printing machine must therefore be stable in terms of its characteristic curve, in order that compensation for printing-unit-specific faults when setting an image can be made by influencing the digital data, and the printing quality does not change in continuous printing either.
However, if inking units have few splitting points, then because of the low layer thickness equalization, an irregular course of the optical density in the printing direction occurs, which is visually perceptible in the case of high quality requirements.
In addition, in an ink supply to the ink applicator roll, if there are changes in the damping solution proportion in the splitting points which lie in the ink stream, ink splitting conditions are changed. If the damping solution proportion in the splitting points in the ink stream change as a result of a change in the damping solution feed by a damping unit, in particular in the case of damping directly into the inking unit, the ink supply has to be readjusted in order to obtain the same ink application to the printing plate. This should be avoided.
If the damping solution proportion in the splitting points of the ink stream changes because of a different quantity of damping solution on the printing plate, caused by the distribution of printing and non-printing proportions of the area, pulsations are produced in the ink stream, by which ghosting is caused.
U.S. Pat. No. 5,580,688 discloses a digital printing process using a short inking unit in which, in order to duplicate an original image, the printing plate has on the surface a layer of a ferroelectric material, which may be polarized differently in very small regions.
The printing plate is polarized by there being an electric DC voltage on an electrode and an electrically conductive layer, for example the metallic roll itself, serving as a mating electrode underneath the ferroelectric material. Conversely, the printing plate may be depolarized again by means of an AC voltage, whose frequency lies far above the resonant frequency of the ferroelectric, or by heating to a temperature above the Curie temperature or, by means of subsequent application of a DC voltage, may be polarized uniformly again.
The function, and also the behavior of ferroelectric material under the action of electric fields, and also in the case of energy supply by means of heat, are extensively described in U.S. Pat. No. 5,580,688. Furthermore, the digital production of an erasable printing plate, on which hydrophobic and hydrophilic regions can be displayed and which contains a material with ferroelectric properties is disclosed by U.S. Pat. No. 5,454,318.